The binding of bacteria to human platelets is a likely central mechanism in the pathogenesis of
infective endocarditis. We have previously found that platelet binding by Streptococcus mitis SF100 is mediated by surface components encoded by a lysogenic bacteriophage, SM1. We now demonstrate that SM1-encoded lysin contributes to platelet binding via its direct interaction with
fibrinogen. Far Western blotting of platelets revealed that
fibrinogen was the major
membrane-associated protein bound by lysin. Analysis of lysin binding with purified
fibrinogen in vitro confirmed that these
proteins could bind directly, and that this interaction was both saturable and inhibitable. Lysin bound both the Aalpha and Bbeta chains of
fibrinogen, but not the gamma subunit. Binding of lysin to the Bbeta chain was further localized to a region within the
fibrinogen D fragment. Disruption of the SF100 lysin gene resulted in an 83+/-3.1% reduction (mean +/- SD) in binding to immobilized
fibrinogen by this mutant strain (PS1006). Preincubation of this isogenic mutant with purified lysin restored
fibrinogen binding to wild type levels. When tested in a
co-infection model of
endocarditis, loss of lysin expression resulted in a significant reduction in virulence, as measured by achievable bacterial densities (CFU/g) within vegetations, kidneys, and spleens. These results indicate that bacteriophage-encoded lysin is a multifunctional
protein, representing a new class of
fibrinogen-
binding proteins. Lysin appears to be cell wall-associated through its interaction with
choline. Once on the bacterial surface, lysin can bind
fibrinogen directly, which appears to be an important interaction for the pathogenesis of
endocarditis.